Hot stamped body

ABSTRACT

Provided is a hot stamped body which includes a base metal, and a plated layer formed on a surface of the base metal, wherein the plated layer includes an interface layer, an intermediate layer, and an oxide layer in order from a base metal side, the interface layer contains one or more kinds of Fe—Al alloy, a total area fraction of the Fe—Al alloy being 99% or more, the intermediate layer  22  contains one or more kinds of Fe—Al—Zn alloy, a total area fraction of the Fe—Al—Zn phase being 50% or more, an average composition of the intermediate layer contains, in mass %, Al: 30 to 50% and Zn: 15 to 30%, and an average film thickness of the oxide layer is 3.0 μm or less, and Mg content in the oxide layer is 0.05 to 0.50 g/m 2 .

TECHNICAL FIELD

The present invention relates to a hot stamped body.

BACKGROUND ART

Structural members (formed bodies) used for automobiles or the like maybe produced by performing hot stamping (hot pressing) so as to increaseboth strength and dimensional accuracy. In producing a formed body byperforming hot stamping, a steel sheet is heated to the Ac₃ point orabove, and is rapidly cooled while being subjected to pressing by presstooling. That is, in this production process, pressing and quenching areperformed simultaneously. By performing hot stamping, it is possible toproduce a formed body having high dimensional accuracy and highstrength.

However, a formed body produced by performing hot stamping has beensubjected to a high temperature and hence, scale is formed on thesurface. Accordingly, a technique is proposed in which a plated steelsheet is used as a hot stamping steel sheet so that formation of scaleis suppressed and, further, corrosion resistance is enhanced (see PatentDocuments 1 to 3).

For example, Patent Document 1 discloses a steel sheet for hot pressinghaving a Zn plated layer. Patent Document 2 discloses an aluminum platedsteel sheet for high strength automobile component having an Al platedlayer. Further, Patent Document 3 discloses a Zn-based plated steelmaterial for hot pressing where various elements, such as Mn, are addedinto the plated layer of a Zn plated steel sheet.

LIST OF PRIOR ART DOCUMENTS Patent Document

Patent Document 1: JP2003-73774A

Patent Document 2: JP2003-49256A

Patent Document 3: JP2005-113233A

SUMMARY OF INVENTION Technical Problem

In the technique disclosed in Patent Document 1, Zn remains in an outerlayer of a steel material after hot stamping is performed and hence,high sacrificial anticorrosive action can be expected. However, a steelsheet is worked in a state where Zn is dissolved and hence, there is apossibility that molten Zn enters the steel sheet so that cracks occurin the steel material. This crack is referred to as Liquid MetalEmbrittlement (hereinafter also referred to as “LME”). Fatigueproperties of the steel sheet deteriorate due to LME.

At present, to avoid occurrence of LME, it is necessary to suitablycontrol heating conditions for performing working on a steel sheet. Tobe more specific, a method or the like is adopted where heating isperformed until all molten Zn is diffused in a steel sheet to form Fe—Znsolid solution. However, these methods require long time heating and, asa result, there is a problem that productivity declines.

In the technique disclosed in Patent Document 2, Al having a higherfusing point than Zn is used for a plated layer and hence, differentfrom Patent Document 1, molten metal is less likely to enter a steelsheet. Accordingly, it is predicted that excellent fatigue property canbe obtained and, eventually, the formed body subjected to hot stampingis excellent in fatigue property. However, a steel material on which anAl plated layer is formed has a problem that it is difficult to form aphosphate film at the time of performing phosphate treatment, which isperformed before coating is applied to automobile components. In otherwords, some steel materials may not obtain sufficient phosphatability,thus degrading corrosion resistance after coating.

Further, in the technique disclosed in Patent Document 3, spotweldability is enhanced by modifying an outermost layer (oxide film)after hot stamping is performed. However, depending on an element to beadded, LME still occurs so that there is a possibility that a hot stampsteel material cannot obtain sufficient fatigue property. Further,depending on an element to be added, there is also a possibility thatphosphatability of the steel material is also degraded in addition tofatigue property.

An objective of the present invention, which has been made to overcomethe above-mentioned problems, is to provide a hot stamped body excellentin fatigue property, spot weldability, and corrosion resistance aftercoating.

Solution to Problem

The present invention has been made to overcome the above-mentionedproblems, and the gist of the present invention is the following hotstamped body.

(1) A hot stamped body including: a base metal and a plated layer formedon a surface of the base metal, wherein

the plated layer includes an interface layer, an intermediate layer, andan oxide layer in order from a base metal side,

the interface layer contains an Fe—Al alloy having a microstructurewhich contains one or more kinds selected from αFe, Fe₃Al and FeAl, atotal area fraction of the Fe—Al alloy being 90% or more,

the intermediate layer contains an Fe—Al—Zn phase which contains one ormore kinds selected from Fe(Al, Zn)₂, Fe₂(Al, Zn)₅ and Fe(Al, Zn)₃, atotal area fraction of the Fe—Al—Zn phase being 50% or more,

an average composition of the intermediate layer contains, in mass %,

Al: 30 to 50% and

Zn: 15 to 30%, and

an average film thickness of the oxide layer is 3.0 μm or less, and Mgcontent in the oxide layer is 0.05 to 1.00 g/m².

(2) The hot stamped body described in the above-mentioned (1), wherein

an average film thickness of the interface layer is 1.0 μm or more.

(3) The hot stamped body described in the above-mentioned (1) or (2),wherein

a total content of Al and Zn in the plated layer is 20 to 100 g/m².

(4) The hot stamped body described in any one of the above-mentioned (1)to (3), wherein

a total area fraction of the Fe—Al—Zn phase in the intermediate layer is90% or more.

(5) The hot stamped body described in any one of the above-mentioned (1)to (3), wherein

the plated layer further contains, in mass %, 0.1 to 15% of Si, and

the intermediate layer further contains an Fe—Al—Si phase which containsone kind or two kinds selected from Fe₃(Al, Si) and Fe(Al, Si), a totalarea fraction of the Fe—Al—Zn phase and the Fe—Al—Si phase being 90% ormore.

Advantageous Effects of Invention

According to the present invention, it is possible to obtain a hotstamped body excellent in fatigue property, spot weldability, andcorrosion resistance after coating.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view for describing a structure of a hot stamped bodyaccording to one embodiment of the present invention.

FIG. 2 is one example of an image of a cross section of the hot stampedbody according to one embodiment of the present invention obtained byperforming SEM observation.

DESCRIPTION OF EMBODIMENTS

Inventors of the present invention have conducted studies on a methodfor achieving both of LME resistance at the time of performing hotstamping forming and spot weldability and corrosion resistance aftercoating of a hot stamped body.

First, the inventors of the present invention have conducted studies ona method for enhancing corrosion resistance after coating of a formedbody. As a result, the inventors of the present invention have foundthat corrosion resistance can be enhanced by causing a plated layer ofthe formed body to contain Mg. However, it is found that, in the case ofproducing a formed body whose plated layer contains Mg, LME easilyoccurs at the time of performing hot stamping forming, thusdeteriorating fatigue property. Further, when Mg content in the platedlayer is excessively high, spot weldability is also decreased.

Accordingly, the inventors of the present invention have conductedextensive studies on a method for enhancing corrosion resistance withoutdeteriorating fatigue property and spot weldability. As a result, thefollowing results are obtained. All of the above-mentioned propertiescan be ensured with a good balance by causing a plated layer to adopt astructure including a layer on the base metal side which contains anFe—Al alloy as a main component, an oxide layer on the outer layer side,and a layer positioned between these layers, and by causing anappropriate amount of Mg to be concentrated in the oxide layer formed onthe outer layer.

The present invention is made based on the above-mentioned findings.Hereinafter, the respective requirements of the present invention aredescribed in detail.

(A) Overall Configuration

FIG. 1 is a view for describing the structure of the hot stamped bodyaccording to one embodiment of the present invention. Further, FIG. 2shows one example of an image of the cross section of the hot stampedbody according to one embodiment of the present invention obtained byperforming SEM observation. As shown in FIGS. 1 and 2, the hot stampedbody 1 according to one embodiment of the present invention includes abase metal 10 and a plated layer 20 formed on the surface of the basemetal 10.

(B) Base Metal

Improvement of fatigue property, spot weldability, and corrosionresistance after coating, which is the task for the hot stamped bodyaccording to this embodiment, can be achieved by the configuration ofthe plated layer. Accordingly, the base metal of the hot stamped bodyaccording to this embodiment is not particularly limited. However, whenthe components of the base metal fall within ranges describedhereinafter, it is possible to obtain the formed body having favorablemechanical properties in addition to fatigue property, spot weldability,and corrosion resistance after coating.

The reasons for limiting respective elements are as follows. In thedescription made hereinafter, symbol “%” for content refers to “mass %”.

C: 0.05 to 0.4%

C (carbon) is an element which increases strength of a hot stamped body.When a content of C is excessively low, the above-mentioned effectcannot be obtained. On the other hand, when a content of C isexcessively high, toughness of a steel material decreases. Accordingly,the C content is set to 0.05 to 0.4%. The C content is preferably 0.10%or more, and is more preferably 0.13% or more. Further, the C content ispreferably 0.35% or less.

Si: 0.5% or less

Si (silicon) is an element which is inevitably contained, and has anaction of deoxidizing steel. However, when a content of Si isexcessively high, Si in steel is diffused during heating of a hot stampand hence, oxide is formed on the surface of a steel sheet, thusdegrading phosphatability. Si is also an element which raises the Ac₃point of a steel sheet. When the Ac₃ point is raised, there is apossibility that a heating temperature at the time of performing hotstamping exceeds the evaporation temperature of Zn plating. Accordingly,the Si content is set to 0.5% or less. The Si content is preferably 0.3%or less, and is more preferably 0.2% or less. There is no limitation onthe lower limit value of the Si content in terms of the above-mentionedproperties of a product. However, as described above, Si is used fordeoxidation and hence, there is a substantial lower limit value.Although the lower limit value of the Si content varies according to therequired level of deoxidation, the lower limit value of the Si contentis usually 0.05%.

Mn: 0.5 to 2.5%

Mn (Manganese) is an element which increases hardenability, thusincreasing strength of a steel material on which hot stamping isperformed. When a content of Mn is excessively low, this effect cannotbe obtained. On the other hand, when a content of Mn is excessivelyhigh, this effect is saturated. Accordingly, the Mn content is set to avalue within a range from 0.5 to 2.5%. The Mn content is preferably 0.6%or more, and is more preferably 0.7% or more. Further, the Mn content ispreferably 2.4% or less, and is more preferably 2.3% or less.

P: 0.03% or less

P (phosphorus) is an impurity contained in steel. P segregates atcrystal grain boundaries, thus decreasing toughness of the steel henceleading to degrading delayed fracture resistance. Accordingly, a contentof P is set to 0.03% or less. It is preferable to reduce the P contentas much as possible.

S: 0.01% or less

S (sulfur) is an impurity contained in steel. S forms sulfides, thusdecreasing toughness of the steel hence leading to degrading delayedfracture resistance. Accordingly, a content of S is set to 0.01% orless. It is preferable to reduce the S content as much as possible.

sol. Al: 0.1% or less

Al (Aluminum) is an element which is generally used for deoxidizingsteel, and is inevitably contained. However, when a content of Al isexcessively high, although deoxidation is sufficiently performed, thereis a possibility that the Ac₃ point of a steel sheet is raised so that aheating temperature at the time of performing hot stamping exceeds anevaporation temperature of Zn plating. Accordingly, the Al content isset to 0.1% or less. The Al content is preferably 0.05% or less. Toobtain the above-mentioned advantageous effects, the Al content ispreferably 0.01% or more. In this specification, the Al content meanscontent of sol. Al (acid-soluble Al).

N: 0.01% or less

N (nitrogen) is an impurity which is inevitably contained in steel. Nforms nitrides, thus decreasing toughness of the steel. Further, in thecase where B is contained in steel, N is bonded to B, thus reducing theamount of dissolved B and, eventually, decreasing hardenability.Accordingly, a content of N is set to 0.01% or less. It is preferable toreduce the N content as much as possible.

B: 0 to 0.005%

B (boron) has an effect of increasing hardenability of the steel, thusincreasing strength of a steel material on which hot stamping isperformed. Accordingly, B may be contained when necessary. However, whena content of B is excessively high, this effect is saturated.Accordingly, the B content is set to 0.005% or less. To obtain theabove-mentioned advantageous effects, the B content is preferably0.0001% or more.

Ti: 0 to 0.1%

Ti (titanium) is bonded to N, thus forming nitrides. When Ti and N arebonded to each other in this manner, bonding between B and N issuppressed and hence, it is possible to suppress degrading hardenabilitycaused by the formation of BN. Accordingly, Ti may be contained whennecessary. However, when a content of Ti is excessively high, theabove-mentioned effect is saturated and, further, an excessively largeamount of Ti nitride precipitates, thus decreasing toughness of thesteel. Accordingly, the Ti content is set to 0.1% or less. Ti makes afine austenite grain size at the time of heating by a hot stamp bypinning effect of Ti, thus increasing toughness and the like of thesteel material. To obtain the above-mentioned advantageous effects, theTi content is preferably 0.01% or more.

Cr: 0 to 0.5%

Cr (chromium) has an effect of increasing hardenability of the steel.Accordingly, Cr may be contained when necessary. However, when a contentof Cr is excessively high, Cr carbide is formed. This Cr carbide is noteasily dissolved at the time of heating the hot stamp and hence,austenitization is prevented from easily progressing, thus degradinghardenability. Accordingly, the Cr content is set to 0.5% or less. Toobtain the above-mentioned advantageous effects, the Cr content ispreferably 0.1% or more.

Mo: 0 to 0.5%

Mo (molybdenum) has an effect of increasing hardenability of the steel.Accordingly, Mo may be contained when necessary. However, when a contentof Mo is excessively high, the above-mentioned effect is saturated.Accordingly, the Mo content is set to 0.5% or less. To obtain theabove-mentioned advantageous effects, the Mo content is preferably 0.05%or more.

Nb: 0 to 0.1%

Nb (niobium) forms carbides, thus having an effect of refining grains atthe time of performing hot stamping hence leading to an increase intoughness of the steel. Accordingly, Nb may be contained when necessary.However, when a content of Nb is excessively high, not only that theabove-mentioned effect is saturated, but also that hardenability isdegraded. Accordingly, the Nb content is set to 0.1% or less. To obtainthe above-mentioned advantageous effects, the Nb content is preferably0.02% or more.

Ni: 0 to 1.0%

Ni (nickel) has an effect of increasing toughness of the steel. Further,Ni suppresses embrittlement attributable to the presence of molten Zn atthe time of heating by the hot stamp. Accordingly, Ni may be containedwhen necessary. However, when a content of Ni is excessively high, theseeffects are saturated. Accordingly, the Ni content is set to 1.0% orless. To obtain the above-mentioned advantageous effects, the Ni contentis preferably 0.1% or more.

In the chemical composition of the base metal which forms the hotstamped body of this embodiment, the balance consists of Fe andimpurities. In this embodiment, “impurity” means a component which, inindustrially producing steel materials, may be mixed in ores or scrapforming raw materials, or a component which may be mixed due to aproduction environment or the like, the component not beingintentionally added.

(C) Plated Layer

As shown in FIG. 1, the plated layer 20 in this embodiment includes, inorder from the base metal 10 side, an interface layer 21, anintermediate layer 22, and an oxide layer 23. The respective layers aredescribed in detail. In this specification, an average film thicknessmeans an average value between the maximum film thickness and theminimum film thickness of a target layer (film).

The interface layer 21 is formed adjacently to the base metal 10, andhas a microstructure which contains an Fe—Al alloy as a main component.In the present invention, the term “Fe—Al alloy” is a collective termfor αFe, Fe₃Al and FeAl. That is, the interface layer 21 has amicrostructure which contains one or more kinds selected from αFe, Fe₃Aland FeAl. Further, the description “contains an Fe—Al alloy as a maincomponent” means that the total area fraction of an Fe—Al alloy is 90%or more. The total area fraction of an Fe—Al alloy is preferably 95% ormore, and more preferably 99% or more.

A content of Al in the interface layer 21 is 30% or less in mass %, andthe Al content gradually decreases as a distance from the base metal 10reduces. Forming the interface layer 21 adjacently to the base metal 10can suppress LME. Further, there may be a case where Zn, Si or the likeis dissolved in an Fe—Al alloy and hence, the interface layer 21 maycontain Zn: 10% or less, or Si: 10% or less.

To enhance fatigue property or the like attributable to LME resistance,the average film thickness of the interface layer 21 is preferably 1.0μm or more, and is more preferably 2.0 μm or more. The lower limit ofthe average film thickness of the interface layer 21 is furtherpreferably 5.0 μm, 6.0 μm, or 7.0 μm.

It is unnecessary to specify the upper limit value of the average filmthickness of the interface layer. However, the interface layer 21 havingthe average film thickness of 15.0 μM may deteriorate properties, suchas corrosion resistance, and such an interface layer 21 is notpreferable. Accordingly, the average film thickness of the interfacelayer 21 is preferably 15.0 μm or less. The upper limit of the averagefilm thickness of the interface layer 21 is preferably 12.0 μm, 11.0 μm,or 10.0 μm.

The intermediate layer 22 has a microstructure which contains anFe—Al—Zn phase as a main component. In the present invention, the term“Fe—Al—Zn phase” is a collective term for Fe(Al, Zn)₂, Fe₂(Al, Zn)₅, andFe(Al, Zn)₃. That is, the intermediate layer 22 has a microstructurewhich contains one or more kinds selected from Fe(Al, Zn)₂, Fe₂(Al, Zn)₅and Fe(Al, Zn)₃. Further, the description “contains an Fe—Al—Zn phase asa main component” means that the total area fraction of an Fe—Al—Znphase is 50% or more. When the plated layer contains no Si, the totalarea fraction of an Fe—Al—Zn phase is preferably 90% or more, is morepreferably 95% or more, and is further preferably 99% or more.

On the other hand, as described later, causing the plated layer tocontain Si allows adhesiveness between the base metal and the platedlayer to be enhanced. In this case, the intermediate layer 22 furthercontains an Fe—Al—Si phase. The term “Fe—Al—Si phase” is a collectiveterm for Fe₃(Al, Si) and Fe(Al, Si). That is, the intermediate layer 22further contains one kind or two kinds selected from Fe₃(Al, Si) andFe(Al, Si). In this case, the total area fraction of an Fe—Al—Zn phaseand an Fe—Al—Si phase is preferably 90% or more, is more preferably 95%or more, and is further preferably 99% or more.

Further, the intermediate layer 22 has an average compositioncontaining, in mass %, Al: 30 to 50% and Zn: 15 to 30%.

By setting a content of Al in the intermediate layer 22 to 30% or more,LME can be suppressed, thus enhancing fatigue property. Further, settingthe Al content to 50% or less allows excellent phosphatability to beensured, thus enhancing corrosion resistance after coating. The Alcontent is preferably 32% or more, and is more preferably 35% or more.Further, the Al content is preferably 48% or less, and is morepreferably 45% or less.

Setting a content of Zn in the intermediate layer 22 to 15% or moreallows excellent phosphatability to be ensured, thus enhancing corrosionresistance after coating. Further, by setting the Zn content to 30% orless, LME can be suppressed, thus enhancing fatigue property. The Zncontent is preferably 17% or more, and is more preferably 20% or more.Further, the Zn content is preferably 28% or less, and is morepreferably 25% or less.

Further, reducing a content of Mg in the intermediate layer 22 canenhance LME resistance. Accordingly, the Mg content is preferably 1.0%or less. In the case where the intermediate layer 22 contains anFe—Al—Si phase, the intermediate layer 22 may contain Si: 25% or less.

The limitation is not particularly imposed on the film thickness of theintermediate layer. However, when the intermediate layer has a smallfilm thickness, corrosion resistance property of a formed body isdegraded. Accordingly, it is desirable to set the film thickness of theintermediate layer to 5.0 μm or more. On the other hand, when theintermediate layer has an excessively large film thickness,manufacturing cost increases and, further, there is a possibility that aheating time at the time of performing hot stamp increases. Accordingly,it is desirable that the film thickness of the intermediate layer be30.0 μm or less.

The oxide layer 23 is an oxide layer which contains Zn as a maincomponent, and the oxide layer 23 contains Mg. In this embodiment, thedescription “an oxide layer which contains Zn as a main component”specifically means that 50 mass % or more of a metal component containedin oxide is Zn. Due to the presence of the oxide layer 23,phosphatability is enhanced. However, excessively large thickness of theoxide layer 23 adversely affects corrosion resistance, weldability andthe like of a formed body and hence, the average film thickness of theoxide layer 23 is set to 3.0 μm or less. To enhance properties of thehot stamped body, such as spot weldability and corrosion resistanceafter coating, the average film thickness of the oxide layer 23 ispreferably set to 2.0 μm or less.

Causing the oxide layer 23 to contain Mg allows corrosion resistanceafter coating to be enhanced. To obtain such an effect, a content of Mgin the oxide layer 23 is set to 0.05 g/m² or more. However, Mg oxide hashigh electrical resistance and hence, when the Mg content increases,spot weldability is decreased. Accordingly, to ensure spot weldability,it is necessary to set the Mg content to 1.00 g/m² or less.

To cause oxide of the hot stamped body to contain Mg, Mg may becontained in a plated layer before hot stamping is performed, or a filmwhich contains Mg may be formed on a plated steel sheet in the form ofcoating or the like.

Cr, Ca, Sr, Ti or the like is easily oxidized in the same manner as Mgand hence, Cr, Ca, Sr, Ti or the like is concentrated on the outer layerof a formed body as oxides. Accordingly, the oxide layer 23 may containthese elements. However, these oxides also have high electricalresistance in the same manner as Mg and hence, when these elements areexcessively concentrated, weldability of a hot stamped body may bedeteriorated. Accordingly, the total content of Mg, Cr, Ca, Sr and Ti inthe oxide layer 23 is preferably 2.0 g/m² or less.

Further, the total content of Al and Zn in the plated layer 20 ispreferably 20 to 100 g/m². By setting the total content of Al and Zn to20 g/m² or more, it is possible to obtain advantageous effects broughtabout by forming the plated layer 20 on the surface of the base metal10. On the other hand, by setting the total content to 100 g/m² or less,raw material cost of a hot stamped body can be suppressed, thus reducingmanufacturing cost and, at the same time, weldability of the hot stampedbody can be ensured. The total content is preferably 30 g/m² or more,and the total content is preferably 90 g/m² or less.

It is preferable that the plated layer 20 further contain, in mass %,0.1 to 15% of Si. Setting a content of Si in the plated layer to 0.1% ormore allows adhesiveness between the base metal and the plated layer tobe enhanced. On the other hand, setting the Si content to 15% or lessallows properties of a hot stamped body, such as corrosion resistanceand weldability, to be ensured. The Si content is preferably 0.3% ormore, and the Si content is preferably 10% or less.

The limitation is not particularly imposed on the film thickness of theentire plated layer 20. However, from a viewpoint of ensuring corrosionresistance, it is preferable to set the film thickness of the entireplated layer 20 to more than 6.0 μm. On the other hand, from a viewpointof economic efficiency, it is preferable to set the film thickness ofthe entire plated layer 20 to 48.0 μm or less.

In the present invention, the microstructures, the average compositionsand the thicknesses of the interface layer, the intermediate layer, andthe oxide layer, and the chemical composition of the plated layer areobtained by the following method.

First, a formed body is cut perpendicular to the surface of the formedbody, and the cross section is polished. Then, concentrations ofrespective elements in the region of the interface layer and in theregion of the intermediate layer on the cross section are analyzed withan Electron Probe Micro Analyzer (EPMA). At this point of operation,mapping analysis is performed in a region which extends upward anddownward in the film thickness direction of each layer by 25% or morefrom the film thickness center of the layer, and which extends in thewidth direction by 20 μm or more, and the average composition of theregion is used. With such analysis, the Al content and the Zn content inthe interface layer, and the contents of Al, Zn and Mg in theintermediate layer are measured.

Further, an average Si content in the entire plated layer is obtained bythe following method. First, line analysis is performed by an EPMA at0.2 μm pitch from the base metal side toward the surface side of theplated layer. Then, the average value of the measurement result in theplated layer is obtained, and the obtained value is set as the averagecomposition of the entire plated layer. In performing continuousmeasurement from the base metal side to the surface side of the platedlayer, a portion at which concentration of Fe is lower than averagecomposition of base metal is assumed as one end portion of the platedlayer, a portion at which concentration of Zn of metal componentscontained in the oxide layer becomes less than 50 mass % is assumed asthe other end portion of the plated layer, and the region between oneend portion and the other end portion of the plated layer is assumed asthe plated layer. Further, line analysis is performed at five or moreportions, and the average value of these line analyses is adopted.

The total content of Al and Zn contained in the plated layer can bemeasured such that a hot stamped body is dissolved with hydrochloricacid, and the dissolved solution is subjected to inductively coupledplasma emission spectrometry (ICP spectrometry). With the use of thismethod, the amount of Al and the amount of Zn can be obtainedindividually.

In dissolving a plated steel material before being heated by a hotstamp, to cause only a plated layer to be dissolved, inhibitor whichsuppresses dissolution of Fe in the base metal is generally added to ahydrochloric acid. However, the plated layer of the hot stamped bodycontains Fe and hence, the plated layer of the hot stamped body is notsufficiently dissolved with the above-mentioned method.

Accordingly, when amounts of Al and Zn in plating of the formed body areobtained by ICP spectrometry, it is appropriate to adopt a method wherea plated layer is dissolved at a solution temperature of 40 to 50° C.using a hydrochloric acid to which inhibitor is not added. Further,after the dissolution is performed, it is desirable to performcomposition analysis using an EPMA on the surface of the hot stampedbody after being dissolved so as to check for the presence or absence ofundissolved plating component, such as Al or Zn. The above-mentionedanalysis is required to be performed on an unworked region of the formedbody.

Further, contents of Mg, Cr, Ca, Sr and Ti contained in the oxide layerare measured such that the hot stamped body is dissolved with anammonium dichromate solution, and the dissolved solution is subjected toICP spectrometry. With the use of the above-mentioned solution, only theoxide layer can be dissolved. With the use of this method, the contentof each of Mg, Cr, Ca, Sr or Ti can be obtained individually.

Further, the microstructure of the interface layer and themicrostructure of the intermediate layer can be obtained by performingcrystal structure analysis using a TEM. Further, the thicknesses of theinterface layer, the intermediate layer, and the oxide layer can beobtained such that the above-mentioned cross section is photographed byan SEM, and this microscope photograph is subjected to image analysis.

The configuration of the plated layer of the formed body according tothis embodiment is not substantially uniform along the directionparallel to the surface of the formed body. Particularly, thethicknesses of the interface layer, the intermediate layer and the oxidelayer vary in many cases between a worked region and an unworked region.Accordingly, the above-mentioned analysis is required to be performed onan unworked region of the formed body. A formed body where the state ofan unworked region of the plated layer falls within the above-mentionedrange is assumed as the formed body according to this embodiment.

(D) Production Method

The method for producing a hot stamped body of this embodiment includesa step of producing a hot stamping plated steel material and a step ofperforming hot stamping on the hot stamping plated steel material.Further, the step of producing a hot stamping plated steel materialincludes a step of producing a base metal of the hot stamping platedsteel material, and a step of forming an Al—Zn plated layer on the basemetal of the hot stamping plated steel material. Further, a rustpreventive oil film forming step and a blanking step may be performedbefore the step of performing hot stamping when necessary. Hereinafter,each step is described in detail.

[Base Metal Producing Step]

In the base metal producing step, a base metal of a hot stamping platedsteel material is produced. For example, molten steel is produced whichhas a chemical composition equal to the chemical composition of the basemetal of the hot stamped body according to this embodiment exemplifiedabove. Then, using this molten steel, a slab is produced by a castingprocess, or an ingot is produced by an ingot-making process.

Next, the slab or the ingot is subjected to hot rolling, thus obtaininga base metal (hot-rolled sheet) of the hot stamping plated steelmaterial. It may be possible to adopt the configuration where picklingtreatment is performed on the above-mentioned hot-rolled sheet, and coldrolling is performed on the hot-rolled sheet on which the picklingtreatment is performed, thus obtaining a cold rolled sheet, and thiscold rolled sheet is used as the base metal of the hot stamping platedsteel material.

[Plating Treatment Step]

In the plating treatment step, an Al—Zn—Mg plated layer is formed on thebase metal of the above-mentioned hot stamping plated steel material,thus producing a hot stamping plated steel material. As a method forforming the Al—Zn—Mg plated layer, hot dip plating treatment may beadopted. Alternatively, any other treatment may be adopted such asspraying plating treatment or vapor deposition plating treatment. Toincrease adhesiveness between the base metal and the plated layer, it ispreferable to cause the plated layer to contain Si.

An example of forming the Al—Zn—Mg plated layer by hot dip platingtreatment is as follows. That is, the base metal is immersed into a hotdipping bath consisting of Al, Zn, Mg and impurities to cause a platedlayer to adhere to the surface of the base metal. Next, the base metalto which the plated layer is caused to adhere is pulled up from theplating bath.

As described above, it is preferable that the total content of Al and Znin the plated layer of the hot stamped body be 20 to 100 g/m². To ensurethis total content, it is important to set the total content of Al andZn in the plated layer when the base metal is pulled up from the platingbath to 20 to 100 g/m² in this step.

In this step, by suitably adjusting a speed at which the steel sheet ispulled up from the plating bath and the flow rate of a wiping gas, thetotal content of Al and Zn in the plated layer can be adjusted.

Further, as described above, the intermediate layer of the plated layerof the hot stamped body contains, in mass %, 30 to 50% of Al and 15 to30% of Zn. These contents of Al and Zn can be also controlled mainly inthis step (plating treatment step). To be more specific, in this step,when Al content in the plating bath is set to 40 to 60%, and Zn contentis set to 40 to 60%, it is possible to allow contents of Al and Zn inthe hot stamped body to fall within the above-mentioned ranges.

In the case where the Al—Zn—Mg plated layer is formed by performing hotdip plating treatment, the Mg content in the plating bath is preferablyset to 0.5 to 2.0%, and is more preferably set to 1.0 to 1.5%. Althoughthe situation may vary depending on the adhesion amount on the platedsteel sheet, when a concentration of Mg in the plating bath is high, theamount of Mg contained in plating increases and hence, the amount of Mgcontained in oxide in the outer layer of the formed product increases,whereby there is a possibility that weldability is decreased. Further,when the amount of Mg which remains in the intermediate layer exceeds1.0%, there is also a possibility that LME resistance is decreased. Onthe other hand, when a concentration of Mg in the plating bath is low,the amount of Mg contained in oxide in the outer layer of the formedproduct decreases so that there is a possibility that sufficientcorrosion resistance after coating cannot be obtained.

In the case where a hot dipping bath which contains no Mg is used, Mgmay be applied by coating such that treatment solution which contains Mgoxide is applied by coating on the plated layer by a bar coater, and thetreatment solution is baked and dried by an oven. When Mg is applied bycoating, it is preferable to set the content of Mg to be applied bycoating to 0.050 to 1.00 g/m².

[Hot Stamping Step]

In the hot stamping step, hot stamping is performed on theabove-mentioned hot stamping plated steel material. Normal hot stampingis performed such that a steel material is heated to a temperaturewithin a hot stamping temperature range (hot working temperature range)and, then, the steel material is subjected to hot working and, further,the steel material is cooled. According to a normal hot stampingtechnique, it is preferable to increase a heating speed of a steelmaterial as much as possible so as to shorten a production time.Further, when a steel material is heated to a temperature within a hotstamping temperature range, the plated layer is sufficiently alloyed.Accordingly, in the normal hot stamping technique, an importance is notplaced on control of heating conditions of the steel material.

However, in the hot stamping step for producing the hot stamped bodyaccording to this embodiment, after alloying heat treatment is performedon a hot stamping plated steel material, the hot stamping plated steelmaterial is heated to a hot stamping temperature (quenching heatingtemperature), and is subjected to hot working and cooling. When thetemperature of the hot stamping plated steel material is increased to ahot stamping temperature, alloying heat treatment, where the hotstamping plated steel material is held for a fixed time within apredetermined temperature range, is performed and hence, a plated layerhaving the above-mentioned configuration can be formed.

In the hot stamping step, first, the hot stamping plated steel materialis charged into a heating furnace (gas furnace, electric furnace,infrared furnace or the like). The hot stamping plated steel material isheated to a temperature range from 500 to 750° C. in the heatingfurnace, and alloying heat treatment is performed, where the platedsteel material is held for 10 to 450 s within this temperature range.Performing alloying heat treatment causes Fe in the base metal todiffuse in the plated layer so that alloying process progresses. Due tosuch alloying process, the plated layer is changed to a layer whichincludes an interface layer, an intermediate layer, and an oxide layerin order from the base metal side. An alloying heating temperature isnot necessarily set to a fixed temperature, and may vary within a rangefrom 500 to 750° C.

When an alloying heating temperature is less than 500° C., a speed atwhich a plated layer is alloyed is extremely slow so that a heating timeis extremely elongated and hence, such an alloying heating temperatureis not preferable in terms of productivity. In addition to the above,there is a possibility that the intermediate layer is not sufficientlyformed. On the other hand, when an alloying heating temperature exceeds750° C., growth of an oxide layer is excessively promoted in thistreatment process, thus degrading weldability of the hot stamped body.

Further, when an alloying heating time is less than 10 s, alloyingprocess of the plated layer is not completed and hence, a plated layerincluding the above-mentioned interface layer, intermediate layer, andoxidized layer cannot be obtained. On the other hand, when an alloyingheating time exceeds 450 s, the amount of growth of oxide increasesexcessively, and such a long time leads to declining of productivity.

Limitation is not particularly imposed on heating conditions at the timeof heating a hot stamping plated steel material to the above-mentionedalloying heating temperature. However, a shorter heating time isdesirable in terms of productivity.

After the alloying heat treatment is finished, the hot stamping platedsteel material is heated to a temperature range from the Ac₃ point to950° C. and, then, is subjected to hot working. At this point ofoperation, a time during which the temperature of the hot stampingplated steel material falls within a temperature range (oxidationtemperature range) from the Ac₃ point to 950° C. is limited to 60 s orless. When the temperature of the hot stamping plated steel materialfalls within the oxidation temperature range, the oxidized layer formingthe outer layer of the plated layer grows. When the time during whichthe temperature of the hot stamping plated steel material falls withinthe oxidation temperature range exceeds 60 s, there is a possibilitythat the oxide film excessively grows, thus degrading weldability of theformed body. On the other hand, a speed at which oxide coating is formedis extremely high and hence, the lower limit value of the time duringwhich the temperature of the hot stamping plated steel material fallswithin the oxidation temperature range is more than Os. However, whenthe hot stamping plated steel material is heated in a non-oxidizingatmosphere, such as 100% nitrogen atmosphere, an oxidized layer is notformed. Accordingly, the hot stamping plated steel material is heated inan oxidizing atmosphere, such as an air atmosphere.

Provided that the time during which the temperature of the hot stampingplated steel material falls within the oxidation temperature range is 60s or less, conditions, such as a heating speed and a maximum heatingtemperature, are not particularly defined, and various conditions underwhich hot stamping can be performed may be selected.

Next, the hot stamping plated steel material which is taken out from theheating furnace is subjected to press forming using press tooling. Inthis step, the steel material is quenched by the press toolingsimultaneously with this press forming. A cooling medium (water, forexample) circulates in the press tooling so that the press toolingpromotes heat dissipation of the hot stamping plated steel material andhence, quenching is performed. With the above-mentioned steps, the hotstamped body can be produced.

The description has been made by exemplifying a method which heats a hotstamping plated steel material using a heating furnace. However, the hotstamping plated steel material may be heated by resistance heating. Alsoin this case, the steel material is heated for a predetermined time byresistance heating, and the steel material is subjected to press formingusing press tooling.

[Rust Preventive Oil Film Forming Step]

The rust preventive oil film forming step is a step which is performedafter the plating treatment step and before the hot stamping step, andwhere rust preventive oil is applied by coating to the surface of a hotstamping plated steel material to form a rust preventive oil film. Therust preventive oil film forming step may be arbitrarily included in theproduction method. In the case where a long time is required before hotstamping is performed after a hot stamping plated steel material isproduced, there is a possibility that the surface of the hot stampingplated steel material is oxidized. However, when a rust preventive oilfilm is formed on a hot stamping plated steel material by the rustpreventive oil film forming step, the surface of the hot stamping platedsteel material is not easily oxidized. Accordingly, performing the rustpreventive oil film forming step can suppress the formation of scale onthe formed body. Any known technique may be used as a method for forminga rust preventive oil film.

[Blanking Step]

This step is a step which is performed after the rust preventive oilfilm forming step and before the hot stamping step, and where shearingand/or blanking is performed on the hot stamping plated steel materialto form the steel material into a particular shape. The sheared surfaceof the steel material on which blanking is performed is easily oxidized.However, in the case where a rust preventive oil film is formed on thesurface of the steel material in advance, rust preventive oil expandsalso to the above-mentioned sheared surface to some extent. With suchexpansion of the rust preventive oil, it is possible to suppressoxidization of the steel material on which blanking is performed.

One embodiment of the present invention has been described heretofore.However, the above-mentioned embodiment is for the sake of example ofthe present invention. Accordingly, the present invention is not limitedto the above-mentioned embodiment, and design modifications can be madewhen necessary without departing from the gist of the present invention.

Hereinafter, the present invention is described more specifically withreference to examples. However, the present invention is not limited tothese examples.

Example 1

First, a base metal was prepared. That is, a slab was produced bycontinuous casting process using molten steel having the chemicalcomposition shown in Table 1. Next, the slab was subjected to hotrolling so as to produce a hot rolled steel sheet, and the hot rolledsteel sheet was further subjected to pickling. Thereafter, the hotrolled steel sheet was subjected to cold rolling, thus producing a coldrolled steel sheet. This cold rolled steel sheet was used as a basemetal (sheet thickness: 1.4 mm) for producing a hot stamped body.

TABLE 1 Chemical composition of base metal (mass %, balance is Fe andimpurities) C Si Mn P S sol. Al N B Ti Cr 0.2 0.2 1.3 0.01 0.005 0.020.002 0.002 0.02 0.2

Next, hot stamping plated steel materials (materials No. 1 to 28) wereprepared in accordance with production conditions shown in Table 2 usingthe base metals produced as described above. Further, the time duringwhich each base metal is immersed in the plating bath at the time ofperforming plating treatment was set to 5 s, and a cooling speed atwhich the plated steel material is cooled to 450° C. after being pulledup from the plating bath was set to 10° C./s.

TABLE 2 Plating treatment conditions Coating conditions MaterialComposition of plating bath (mass %) Total content of Al, Zn Mg contentNo. Al Zn Si Mg Cr + Ca + Sr + Ti in the plated layer (g/m²) Mg coating(g/m²) 1 55.0 44.5 0 0.5 0 60 absent 0 2 55.0 44.5 0 0.5 0 80 absent 0 355.0 44.0 0 1.0 0 60 absent 0 4 55.0 44.0 0 1.0 0 75 absent 0 5 55.042.4 1.6 1.0 0 60 absent 0 6 45.0 39.0 15.0 1.0 0 60 absent 0 7 55.044.0 0 2.0 0 40 absent 0 8 55.0 44.0 0 2.0 0 50 absent 0 9 60.0 39.0 01.0 0 60 absent 0 10 50.0 49.0 0 1.0 0 60 absent 0 11 55.0 45.0 0 0 0 60present 0.05 12 55.0 45.0 0 0 0 60 present 0.25 13 55.0 45.0 0 0 0 60present 0.5 14 55.0 45.0 0 0 0 60 present 1 15 55.0 43.0 0 1.0 1 60absent 0 16 55.0 41.4 1.6 1.0 1 60 absent 0 17 55.0 42.0 0 1.0 3 60absent 0 18 55.0 40.4 1.6 1.0 3 60 absent 0 19 20.0 80.0 0 0 0 60 absent0 20 80.0 20.0 0 0 0 60 absent 0 21 0.1 99.9 0 0 0 60 absent 0 22 90.0 010.0 0 0 60 absent 0 23 55.0 45.0 0 0 0 60 absent 0 24 55.0 43.4 1.6 0 060 absent 0 25 55.0 44.9 0 0.05 0 60 absent 0 26 55.0 42.0 0 3.0 0 60absent 0 27 55.0 45.0 0 0 0 60 present 0.01 28 55.0 45.0 0 0 0 60present 1.5

Thereafter, the above-mentioned hot stamping plated steel materials wereheated under conditions (heating No. 1 to 9) shown in Table 3 and,immediately after the heating, were subjected to V-bending simulating ahot stamp using a hand press machine so as to produce hot stamped bodiesof respective test examples. The shape of press tooling is set such thatan outer side portion in the bending radius direction to which V-bendingis applied is extended by approximately 15% at the time when bending isfinished. Further, quenching was performed such that even a portionwhere a cooling speed at the time of performing working is slow has acooling speed of 50° C./s or more until the portion is cooled to anapproximate point (410° C.) at which martensitic transformation starts.

TABLE 3 Alloying Alloying Quenching Quenching heating heating heatingheating Heating temperature time temperature time No. (° C.) (s) (° C.)(s) 1 700 120 900 30 2 500 300 900 30 3 700 300 900 5 4 750 90 900 60 5400 120 900 30 6 700 500 900 30 7 800 90 900 60 8 700 60 900 120 9 — 900180

From the flat plate portion of the obtained hot stamped body of eachtest example, a test piece for observing the structure of the platedlayer, a test piece for ICP spectrometry, a test piece for spotweldability evaluation test, and a test piece for corrosion resistanceafter coating evaluation test were cut out. Further, a test piece forLME resistance evaluation test was cut out from a portion to whichbending is applied.

With respect to the test piece for observing the structure of the platedlayer, the cross section perpendicular to the surface of the formed bodywas polished and, thereafter, the contents of Al and Zn in the interfacelayer and the contents of Al, Zn and Mg in the intermediate layer weremeasured using an EPMA. In EPMA analysis, mapping analysis was performedin a region which extends upward and downward in the film thicknessdirection of each layer by 25% or more from the film thickness center ofthe layer, and which extends in the width direction by 20 μm or more,and the average composition in the region was calculated.

Further, in obtaining the average Si content in the entire plated layer,line analysis was performed by an EPMA at 0.2 μm pitch from the basemetal side toward the surface side of the plated layer, and the averagevalue of the measurement result of the plated layer was calculated. Lineanalysis was performed at five portions, and the average value of theline analyses was used as the average composition of the entire platedlayer.

Further, the above-mentioned cross section was photographed by an SEM,and the microscope photograph was subjected to image analysis so as tomeasure the thickness of each layer. The microstructure of each layerwas determined by performing crystal structure analysis with a TEM on athin piece obtained from the same place of each test piece.

With respect to the test piece for ICP spectrometry, a plated layer wasdissolved with hydrochloric acid at a temperature of 50° C. and,thereafter, the dissolved solution was subjected to ICP spectrometry soas to obtain the total content of Al and Zn contained in the platedlayer. Further, in the same manner, only the oxide layer of the testpiece for ICP spectrometry was dissolved with an ammonium dichromatesolution, and the dissolved solution was subjected to ICP spectrometryso as to obtain the contents of Mg, Cr, Ca, Sr and Ti.

Next, LME resistance evaluation test, spot weldability evaluation test,and corrosion resistance after coating evaluation test were performed asdescribed below.

[LME Resistance Evaluation Test]

With respect to the cross section in the thickness direction of the testpiece for LME resistance evaluation test of each test example, areflected electron image was observed using an SEM and a reflectedelectron detector so as to observe the presence or absence of occurrenceof LME. At this point of operation, the case where crack propagates to abase metal (a portion where a concentration of Fe is 98% or more) isassumed as occurrence of LME. Further, a test piece with no occurrenceof cracks is evaluated as excellent (1), and a test piece with crackswhich extend beyond the plated layer to a base metal is evaluated asfail (4).

When it is difficult to determine an end position of cracks with theabove-mentioned observation, energy dispersive X-ray spectroscopy (EDS)is performed on a region around the end position of cracks using anenergy dispersive X-ray microanalyzer so as to determine whether or notcracks extend to the base metal. In such an operation, a region wheretotal content of Al and Zn exceeds 0.5% is identified as a plated layer,and a region of a steel material on the inner side of such a region isidentified as a base metal.

[Spot Weldability Evaluation Test]

Spot welding was performed on the test piece for weldability evaluationtest of each test example using a DC power source at an applied pressureof 350 kgf. Tests were performed at various welding currents. A value ofwelding current at which the nugget diameter of a welding portionexceeds 4.7 mm was set to the lower limit value. A value of weldingcurrent was suitably increased, and a value of welding current at whichdust is generated during welding was set to the upper limit value.Values between the upper limit value and the lower limit value are setas the proper current range, and the difference between the upper limitvalue and the lower limit value was used as an index of spotweldability. In the evaluation of spot weldability, a test piece withthis value of 1.5 A or more is evaluated as excellent (1). A test piecewith this value of 1.0 A or more and less than 1.5 A is evaluated asgood (2). A test piece with this value of 0.5 A or more and less than1.0 A is evaluated as fair (3). A test piece with this value of lessthan 0.5 A is evaluated as fail (4).

[Corrosion Resistance after Coating Evaluation Test]

Surface conditioning was performed on the test piece for corrosionresistance after coating evaluation test of each test example for 20 sat a room temperature using a surface conditioning agent (product name:PREPALENE X) made by Nihon Parkerizing Co., Ltd. Next, phosphatetreatment was performed using a zinc phosphate treatment solution(product name: PALBOND 3020) made by Nihon Parkerizing Co., Ltd. To bemore specific, the temperature of the treatment solution was set to 43°C., and the formed body was immersed into the treatment solution for 120s. With such operations, a phosphate coating was formed on the surfaceof the steel material.

After the above-mentioned phosphate treatment was performed, cationicelectrodeposition paint made by NIPPONPAINT Co., Ltd. was applied toeach formed body by electrodeposition coating by slope energization at avoltage of 160 V and, further, was subjected to baking coating for 20minutes at a baking temperature of 170° C. Control of the film thicknessof the paint after the electrodeposition coating was performed underconditions that electrodeposition coating on a steel material before hotstamping forming is performed has a thickness of 15 μm.

A cross-cut was made on the formed body on which electrodepositioncoating was performed such that the cross-cut reaches the steel materialwhich is a base metal, and a composite corrosion test (JASO M610 cycle)was performed. Corrosion resistance was evaluated based on the width ofcoating blister. After a composite corrosion test of 180 cycles isperformed on a formed body, the formed body with a width of coatingblister of 2.0 mm or less is evaluated as excellent (1), the formed bodywith a width of coating blister of more than 2.0 mm and 3.0 mm or lessis evaluated as good (2), the formed body with a width of coatingblister of more than 3.0 mm and 4.0 mm or less is evaluated as fair (3),and the formed body with a width of coating blister of more than 4.0 mmis evaluated as fail (4).

[Evaluation Result]

It is an objective of the present invention to provide a hot stampedbody excellent in all of fatigue property (LME resistance), spotweldability, and corrosion resistance after coating with a good balance.Accordingly, by comprehensively taking these evaluation results intoaccount, a hot stamped body which has an evaluation of excellent or goodin either test, thus having a comprehensive evaluation of “A” and a hotstamped body which does not have an evaluation of fail in either test,thus having a comprehensive evaluation of “B” are assumed as acceptable.A hot stamped body which has an evaluation of fail in either test, thushaving a comprehensive evaluation of “C” is assumed as defective. Theseresults are shown in Table 4.

TABLE 4 Interface layer Intermediate layer Oxide layer Average AverageCr + Ca + composition Film composition Film Sr + Ti Test MaterialHeating (mass %) Structure thickenss mass % Structure thickenss Mgcontent content No. No. No. Al Zn Judgement (μm) Al Zn Mg Judgement (μm)(g/m²) (g/m²) 1 1 1 15 3 OK 10 40 18 0 OK 20 0.30 0 2 2 1 17 5 OK 8 4220 0 OK 30 0.40 0 3 3 1 15 3 OK 10 40 18 0 OK 20 0.60 0 4 4 1 16 4 OK 841 19 1.0 OK 27 0.75 0 5 5 1 15 3 OK 10 40 18 0 OK 20 0.60 0 6 6 1 15 3OK 10 40 18 0 OK 20 0.60 0 7 7 1 14 3 OK 12 40 15 1.0 OK 16 0.80 0 8 8 114 3 OK 12 40 15 0 OK 18 1.0  0 9 9 1 15 3 OK 10 43 17 0 OK 22 0.60 0 1010 1 15 3 OK 10 40 19 0 OK 22 0.60 0 11 11 1 15 3 OK 10 40 18 0 OK 200.05 0 12 12 1 15 3 OK 10 40 18 0 OK 20 0.25 0 13 13 1 15 3 OK 10 40 180 OK 20 0.50 0 14 14 1 15 3 OK 10 40 18 0 OK 20 1.0  0 15 5 1 15 3 OK 1040 18 0 OK 20 0.60 0 16 5 2 15 3 OK 10 40 18 0 OK 20 0.60 0 17 5 3 20 5OK 5 42 22 0 OK 20 0.60 0 18 5 4 15 3 OK 12 40 16 0 OK 20 0.60 0 19 15 115 3 OK 10 40 18 0 OK 20 0.60 0.60 20 16 1 15 3 OK 10 40 18 0 OK 20 0.600.60 36 17 1 15 3 OK 10 40 18 0 OK 20 0.60 1.8 37 18 1 15 3 OK 10 40 180 OK 20 0.60 1.8 21 19 1 15 10 OK 10 20 45 0 NG 18 0   0 22 20 1 25 1 OK10 40 10 0 NG 25 0   0 23 21 1 0 40 NG 5  0 40 0 NG 17 0   0 24 22 1 200 OK 10 40  0 0 NG 25 0   0 25 23 1 15 3 OK 10 40 25 0 OK 20 0   0 26 241 15 3 OK 10 40 25 0 OK 20 0   0 27 25 1 15 3 OK 10 40 25 0 OK 20 0.03 028 26 1 15 3 OK 10 40 25 1.2 OK 20 1.2  0 29 27 1 15 3 OK 5 40 18 0 OK20 0.01 0 30 28 1 15 3 OK 5 40 18 0 OK 20 1.5  0 31 5 5 20 3 NG 0.5 4525 0 OK 20 0.40 0 32 5 6 15 3 OK 15 38 12 0 OK 20 0.60 0 33 5 7 15 3 OK15 38 12 0 OK 20 0.60 0 34 5 8 15 3 OK 10 35 12 0 OK 22 0.60 0 35 5 9 153 OK 10 35 12 0 OK 22 0.60 0 Entire plated layer Evaluation result Oxidelayer Average Fatigue Film composition Al, Zn total property CorrosionTest thickenss (mass %) content (LME Spot resistance Comprehensive No.(μm) Si (g/m²) resistance) weldability after coating evaluation  1 2.0 057 1 2 2 A Inventive  2 2.5 0 77 1 2 1 A example  3 2.0 0 57 1 2 2 A  42.5 0 72 1 2 1 A  5 2.0 1 57 1 2 2 A  6 2.0 8 58 1 2 2 B  7 2.0 0 38 1 22 A  8 2.0 0 48 1 2 2 A  9 2.0 0 57 1 2 2 A 10 2.0 0 57 1 2 2 A 11 1.5 058 1 2 3 B 12 1.5 0 58 1 2 2 A 13 1.5 0 58 1 2 2 A 14 1.5 0 58 1 3 2 B15 2.0 0 57 1 2 2 A 16 2.0 0 57 1 2 2 A 17 1.5 0 58 1 2 2 A 18 2.5 0 561 2 2 A 19 2.0 0 57 1 2 2 A 20 2.0 1 57 1 2 2 A 36 2.0 0 57 1 3 2 B 372.0 1 57 1 3 2 B 21 2.0 0 58 4 3 3 C Comparative 22 1.5 0 59 1 2 4 Cexample 23 2.0 0 56 4 3 3 C 24 0.1 5 60 1 2 4 C 25 2.0 0 57 1 1 4 C 262.0 1 57 1 1 4 C 27 2.0 0 57 1 2 4 C 28 2.0 0 57 4 4 1 C 29 1.5 0 57 1 24 C 30 1.5 0 57 1 4 2 C 31 2.0 1 58 4 2 2 C 32 3.5 1 57 1 3 4 C 33 4.0 156 1 4 4 C 34 3.5 1 56 1 4 3 C 35 4.0 1 55 1 4 3 C

As can be clearly understood from Table 4, it is confirmed that the hotstamped bodies according to the present invention are excellent in allof fatigue property (LME resistance), spot weldability, and corrosionresistance after coating with a good balance.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to obtain a hotstamped body excellent in fatigue property, spot weldability, andcorrosion resistance after coating. Accordingly, the hot stamped bodyaccording to the present invention can be favorably used for astructural member or the like used in an automobile or the like.

1. A hot stamped body comprising: a base metal and a plated layer formedon a surface of the base metal, wherein the plated layer includes aninterface layer, an intermediate layer, and an oxide layer in order froma base metal side, the interface layer contains an Fe—Al alloy having amicrostructure which contains one or more kinds selected from αFe, Fe₃Aland FeAl, a total area fraction of the Fe—Al alloy being 90% or more,the intermediate layer contains an Fe—Al—Zn phase which contains one ormore kinds selected from Fe(Al, Zn)₂, Fe₂(Al, Zn)₅ and Fe(Al, Zn)₃, atotal area fraction of the Fe—Al—Zn phase being 50% or more, an averagecomposition of the intermediate layer contains, in mass %, Al: 30 to 50%and Zn: 15 to 30%, and an average film thickness of the oxide layer is3.0 μm or less, and Mg content in the oxide layer is 0.05 to 1.00 g/m².2. The hot stamped body according to claim 1, wherein an average filmthickness of the interface layer is 1.0 μm or more.
 3. The hot stampedbody according to claim 1, wherein a total content of Al and Zn in theplated layer is 20 to 100 g/m².
 4. The hot stamped body according toclaim 1, wherein a total area fraction of the Fe—Al—Zn phase in theintermediate layer is 90% or more.
 5. The hot stamped body according toclaim 1, wherein the plated layer further contains, in mass %, 0.1 to15% of Si, and the intermediate layer further contains an Fe—Al—Si phasewhich contains one kind or two kinds selected from Fe₃(Al, Si) andFe(Al, Si), a total area fraction of the Fe—Al—Zn phase and the Fe—Al—Siphase being 90% or more.
 6. The hot stamped body according to claim 2,wherein a total content of Al and Zn in the plated layer is 20 to 100g/m².
 7. The hot stamped body according to claim 2, wherein a total areafraction of the Fe—Al—Zn phase in the intermediate layer is 90% or more.8. The hot stamped body according to claim 3, wherein a total areafraction of the Fe—Al—Zn phase in the intermediate layer is 90% or more.9. The hot stamped body according to claim 6, wherein a total areafraction of the Fe—Al—Zn phase in the intermediate layer is 90% or more.10. The hot stamped body according to claim 2, wherein the plated layerfurther contains, in mass %, 0.1 to 15% of Si, and the intermediatelayer further contains an Fe—Al—Si phase which contains one kind or twokinds selected from Fe₃(Al, Si) and Fe(Al, Si), a total area fraction ofthe Fe—Al—Zn phase and the Fe—Al—Si phase being 90% or more.
 11. The hotstamped body according to claim 3, wherein the plated layer furthercontains, in mass %, 0.1 to 15% of Si, and the intermediate layerfurther contains an Fe—Al—Si phase which contains one kind or two kindsselected from Fe₃(Al, Si) and Fe(Al, Si), a total area fraction of theFe—Al—Zn phase and the Fe—Al—Si phase being 90% or more.
 12. The hotstamped body according to claim 6, wherein the plated layer furthercontains, in mass %, 0.1 to 15% of Si, and the intermediate layerfurther contains an Fe—Al—Si phase which contains one kind or two kindsselected from Fe₃(Al, Si) and Fe(Al, Si), a total area fraction of theFe—Al—Zn phase and the Fe—Al—Si phase being 90% or more.